1. Field of the Invention
The present invention relates generally to a wireless communication network, and, in particular, to a method and system for detecting a hidden station in a wireless communication network.
2. Description of the Related Art
Of the standards for wireless communication networks, the Institute of Electrical and Electronics Engineers (IEEE) 802.11 family of standards defines a hidden station. Assuming that a first station and a second station are performing communication with each other, and a third station, which is an arbitrary station, is observing the communication. The third station receives Medium Access Control (MAC) Protocol Data Units (PDUs) from the second station. The PDUs have a MAC address of the second station as a Transmitter Address (TA), and have a MAC address of the first station as a Receiver Address (RA). In addition, the third station receives PDUs from the first station. The PDUs have a MAC address of the first station as a TA, and has a MAC address of the second station as an RA.
However, it is possible that the third station has received a PDU from the second station but has failed to receive a PDU from the first station. In this situation, the third station can perceive the presence of the first station. In this situation, the first station is a hidden station for the third station.
Commonly, if a MAC frame including a MAC address is generated in a MAC layer, the MAC frame is transferred to a physical (PHY) layer, which is a lower layer, for transmission. In the PHY layer, a PHY PDU (PPDU), which is a basic PHY frame including the MAC frame, is generated and then transmitted through wireless resources. For purposes of providing a description herein, a format of the basic frame specified in the IEEE 802.11a standard will be utilized as an example of the structure of the basic PHY frame.
FIG. 1 is a diagram illustrating a structure of a basic PHY frame. Referring to FIG. 1, a Physical Layer Convergence Procedure (PLCP) preamble is comprised of a predetermined number of OFDM symbols, and is used in a receiver for detection of a received frame.
A PLCP Header corresponding to a Signal field includes a 4-bit Rate field, a 1-bit Reserved field, a 12-bit Length field, a 1-bit Parity field, a 6-bit Tail field, and a 16-bit Service field, and is modulated by Binary Phase Shifting Keying (BPSK) and comprised of one OFDM symbol encoded with a ½-rate convolutional code. Herein, the fields containing information important to the present invention include the Rate field indicating a data rate at which a PHY Service Data Unit (PSDU) including a MAC frame is transmitted, and the Length field of the MAC frame or the PDSU. The basic frame structure follows the standard, and a description of the fields other than the foregoing fields is known to those of skill in the art.
Meanwhile, a range where a frame can be transmitted is determined according to a frame data rate. A frame transmitted at a lower data rate can be transferred farther. Because the PLCP Header is generally transmitted at a lower data rate compared with the data or the PDSU, a range (header transfer range) where the PLCP Header can be transferred is greater than a range (data transfer range) where the data can be transferred.
Therefore, a description of hidden station detection according to the definition given in the standard can be given as follows, based on an assumption that a first station receives and decodes not only the PLCP Header but also the whole data frame. After the decoding, the first station recognizes from the decoded data frame that an RA of the frame is indicated as a hardware address, i.e. MAC address, of a second station. However, if the first station has never received a data frame having a MAC address of the second station as a TA, the first station regards the second station as a hidden station.
However, the first station can perceive use/nonuse of a corresponding wireless medium even with the use of the Rate field and the Length field included in the PLCP Header in order to detect a hidden station. That is, the first station has no need to detect a MAC address of a transmitter or a receiver, which it can obtain by receiving and decoding a data frame that cannot be transferred far as it is transmitted at a high data rate. Therefore, there is a need to newly define a hidden station and to efficiently detect the presence/absence of the hidden station.